Ring resonator array with lasing on edge

In the topological-insulator laser demonstrated by the Technion-CREOL team, laser light goes around the perimeter unobstructed by sharp corners or other local disorder, and eventually exits through the output port. [Image: S. Wittek (CREOL) and M.A. Bandres (Technion)]

In recent years, topological insulators—materials that, by virtue of their topological order, are insulating in bulk but conductive on their edges—have been one of the hottest topics of condensed-matter physics. The concept has found analogs in photonics as well, in materials that, for example, have a photonic band gap in bulk but can transmit light in one direction on their edges.

Now, a pair of papers from a research group in Israel and the United States report the design and demonstration of a semiconductor laser based on topological-insulator principles (Science, doi: 10.1126/science.aar4003, 10.1126/science.aar4005). The researchers believe that their results “provide a route for developing a novel class of active topological photonic devices” that are highly efficient and robust to disorder and defects.

Toward “topological protection”

One particularly attractive feature of systems such as topological insulators is the “topological protection” of their edge states. What that means is that the conducting edges of topological insulators are particularly robust against defects and disorders—allowing, for example, scatter-free conduction of electrons in one direction around the edges of the system, despite defects such as sharp corners.

Such robust, scatter-free propagation could be particularly valuable in a system such as a laser, in which imperfections and defects can lead to low output coupling, multimode behavior and reduced efficiency. And recent experiments have indeed demonstrated topologically protected behavior in some photonic setups. But those demos have generally been in passive, conservative systems, not in active systems that experience gain, such as a laser. And the experiments that have been done toward lasing in a topologically protected system have required the action of a magnetic field to nudge the system into topological-insulator-like behavior—a need that could limit practical application in semiconductor lasers.

Microring lattice

The research team behind the current work—led by OSA Fellow Mordechai Segev of The Technion, Israel, and by OSA Fellow Demetrios N. Christodoulides and OSA Life Member Mercedeh Khajavikhan of CREOL, the University of Central Florida, USA—wanted to see if a laser could be built on topological-insulator principles, and in a magnet-free, all-dielectric platform. To find out, they began by numerically designing and testing arrays of resonators under several configurations.

One of those configurations involved a lattice of micro-ring resonators with aperiodic couplers, an architecture that, according to the researchers, “can be implemented using standard semiconductor technology,” without external magnetic fields or exotic materials. (Instead, the combination of aperiodic coupling and the interaction of the clockwise and counterclockwise modes in the rings leads to an artificial magnetic field in the material itself.) Numerical simulations with the configuration showed that it should enable highly efficient, one-way, single-mode lasing around the edges of the system, even when disorders were introduced.

For a real-world test, the team next used the design to fabricate a 2-D, 10×10 square lattice of coupled ring resonators on a platform of 30-nm-thick InGaAsP quantum wells. They tied the configuration to an output-coupling waveguide, and then pumped the perimeter of the array with 10-ns pulses from a 1064-nm laser, using an infrared camera to capture the structure of the lasing modes in the edges of the array.

Robust edge-mode lasing

The team found that the string of microresonator rings on the edge of the array did indeed lase in unison, in “an extended, topologically protected scatter-free edge mode” analogous to the edge current in a topological insulator (see illustration above). The single-mode lasing that resulted was far more efficient than that of a non-topologically-ordered case that the team set up for comparison.

The researchers even introduced some defects in the edge configuration, removing two microring gain elements from the array. They found that the light was able to bypass the defects by taking a quick detour into the non-active, bulk material, and then to continue propagating efficiently around the edge—a clear sign that topological protection was at work in the device.

The Technion’s Segev, in a press release, said that the system goes “against all common knowledge about topological insulators,” the unique robustness of which, he noted, was previously thought to fail if the system contained gain, “as all lasers must have.” The demonstration, he said, suggests that harnessing topological design principles could thus lead to lasers that are “much more efficient, more coherent, and at the same time immune to all kinds of fabrication imperfections, defects and the like.” And that, the team believes, could “pave the way toward a novel class of active topological photonic devices that may be integrated with sensors, antennas and other photonic devices.”